A 12-ton bridge which is 44 feet long is not at all a problem. Even 14-wheeler (6-wheel prime mover and 8-wheel trolley) can haul it effortlessly, plus any 10- wheeler crane could lift it. You know what, these things aren't exactly in the middle of a bazaar. The place is usually somewhat remote and houses' windows are also distant from the track. And suppose you were passing by and saw such a crane operation underway, would you ever think that these are not Pakistan Railway personnel but some thieves? Would it ever occur to anybody to go and confirm their identity?
Just suppose a bus stop. 1 motorcyclist is talking to a bus commuter. He may've asked directions to somewhere or just asking the time. Would it ever occur to somebody that a mobile phone mugging is going on? Or somebody is driving a car on the road: How can anybody else know/suspect if s/he is the rightful owner or a car-lifter?